I'm working on talking Shannon into posting, but until then here are how some of these issues have affected us.
I wish we could offer a lot of help, but....
Shannon has had to write off a chunk of her family because of their extreme religious viewpoints (however most of them were already distant).
Her direct family isn't religious so we didn't get hit with the abomination, but she did get a lot of pressure to divorce immediately before it caused social damage. In the end she had to tell her mother point blank that if she had to decide between them and me that they weren't going to win before they backed off. Having to play that card is terrifying and it may mean the end of the family relationship, but in our case it helped her mother understand that this was real and now after six months she is a strong ally instead of a detractor.
We can't help with the custody aspect, but I do wonder if that is as much as worry as it would have been even a year or two ago. As time continues to pass, society is gradually getting more accepting.
As far as your grandmother goes, think about this. Because of this struggle you have decided to stand with her. Sure there are more forces working to tear you apart, but you are entering this fully committed and not just along for the ride until the car breaks down. Doesn't that help your odds to be at least as good as if she wasn't transitioning, but was still isolated? Ask your grandmother to focus on the positive instead of dwelling on the negative. We create the world we live in and a negative mind creates all sorts of problems.
And finally there is always this:
C-3PO: Sir, the possibility of successfully navigating an asteroid field is approximately 3,720 to 1.
Han Solo: Never tell me the odds.